Is my engine original

My father-in-law just bought a 71 Olds 442. We have checked the Vin # and it is in fact a true 442. It has a 455 engine and appears to be original. My question is how can I tell if the engine in the car is in fact the original engine? I apologizes I do not know a lot about cars, but I would appreciate thoughts or comments.

Look for a casting date on the block, see if is late 70 or 71, then you will need casting number info also to determine what it may be, some Olds 442 clubs on the net may be helpful.

Welcome to AF.
As I recall, unlike Chevrolet and Pontiac, there is no way to tell with Oldsmobiles if the engine block is an exact match to the chassis. Such information was not recorded on Olds blocks. So therefore, there really is no way to prove any Olds engine is original.

The best you can do is to confirm if the engine number was typically installed on '71 442's. However, there is no way to determine if the engine was swapped with another '71 455 block.

The engine number will also tell you if the engine is really a 455. The 400/403/425 blocks looked much the same externally.

Look here for more info. http://www.442.com/oldsfaq/ofblk.htm#BB%201965%20-%201976%20Identifying%20a%20Big%20Block%20Block